Two known methods conventionally used to produce aluminum nitride powders: are (1) firing a mixed composition of alumina and carbon powders in a nitrogen-containing atmosphere as described in Japanese Patent Application (OPI) No. 50008/84 corresponding to U.S. Pat. No. 4,618,592 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application open to public inspection"), and (2) nitriding aluminum by contacting it with a nitrogen gas as described in Japanese Patent Application (OPI) No. 161314/85.
Aluminum nitride powder is useful as the material of choice for manufacturing products such as electrical substrates that require high thermal conductivity. The thermal conductivities of such products are chiefly governed not by free electrons in metals, but by phonons due to lattice vibrations. The degree of phonon conduction depends on what impurities are present in the aluminum nitride powder and on the presence of pores in a sintered body. For instance it is known that thermal conductivity is badly affected not only by the presence of cations such as sodium, iron and silicon ions but also by the inclusion of oxygen from unreacted alumina and other sources. If the powder comprises large particles, it is insufficiently sintered and a substantial amount of pores remains in the sintered product. The presence of such residual pores decreases thermal conductivity determinatively. Therefore, in order to attain high thermal conductivity in electrical substrates and other heat-dissipating products, the powder used as a raw material is required not only to contain few cation impurities but also to comprise small particles which can be easily sintered.
The two prior art methods described above have the disadvantage that the aluminum nitride produced has a tendency to contain residual cationic impurities such as sodium, iron and silicon ions if it is produced by the method described in Japanese Pat. Application (OPI) No. 50008/84 corresponding to U.S. Pat. NO. 4,618,592 and residual cationic impurities such as silicon, iron and magnesium ions if it is produced by the method described in Japanese Patent Application (OPI) No. 161314/85. In addition, the aluminum nitride produced by the former method is liable to contain residual unreacted alumina whereas the product obtained by the latter method has an increased content of total oxygen. The aluminum nitride powders containing such impurities are not suitable for use as materials for manufacturing products such as electrical substrates that require a high degree of purity.
The present inventors conducted intensive studies in order to develop a process for producing an aluminum nitride powder which has low impurity contents and which comprises particles of a small size.